1. Field
The embodiments relate to a receiving apparatus for receiving a modulation signal, and may be applied to, for example, a PSK demodulating apparatus having an equalizer for correcting a distortion of a signal.
2. Description of the Related Art
PSK (Phase Shift Keying) has been widely put into practice use as one of digital modulation methods. PSK is a modulation method for changing the phase of a carrier wave in accordance with transmission data, and widely used in fields such as a satellite communication, etc.
FIG. 1 is a block diagram showing a configuration of a general PSK demodulating apparatus. The PSK demodulating apparatus receives and demodulates a PSK (BPSK, QPSK, 8PSK, etc.) modulation signal.
A quadrature detecting circuit 1 obtains I-Q channel signals of the PSK modulation signal from an RF input signal. I-channel signal and Q-channel signal are respectively digitized and output. A timing recovery circuit 2 recovers the symbol timing of the I-Q channel signals. A carrier recovery circuit 3 recovers a carrier by correcting the frequency shifts of received signals. An FIR equalizer 4 corrects the distortions of the I-Q channel signals. Here, the timing recovery circuit 2 and the carrier recovery circuit 3 can respectively recover timing and a carrier by using signals the distortions of which are corrected by the FIR equalizer 4.
Patent Document 1 (Japanese Patent Publication No. 2002-158724) discloses a PSK demodulating apparatus comprising the following circuits. A digital signal generating circuit generates a digital signal corresponding to a phase axis by performing an analog-to-digital conversion after the coherent detection of a modulated input signal. A frequency correction value outputting circuit outputs a frequency correction value set based on a symbol rate. A frequency correcting circuit generates a frequency-corrected signal by applying a frequency offset to the digital signal on the basis of the frequency correction value. A timing recovery circuit recovers timing by extracting the symbol timing of the frequency-corrected signal. A C/N detecting circuit detects C/N from the symbol obtained by the timing recovery circuit. An optimum frequency correction value determining circuit outputs as an optimum frequency correction value a frequency correction value in the case where the C/N is the highest. A carrier recovery circuit recovers a carrier by correcting the frequency shift of the signal the frequency of which is corrected with the optimum frequency correction value, and the timing of which is recovered. A coherent detection circuit corrects an error of the symbol after the carrier is recovered, and detects a unique word.
In the PSK demodulating apparatus shown in FIG. 1, the FIR equalizer 4 sometimes diverges if a frequency shift (here, a difference between the frequency of the carrier wave and that of a sinusoidal wave used for quadrature detection in the demodulating apparatus) is large. One solution to this problem is, for example, a procedure for suspending the FIR equalizer 4 at the start of reception as represented by the flowchart shown in FIG. 2. Namely, in step S101, the timing recovery circuit 2 and the carrier recovery circuit 3 are operated, and at the same time, the FIR equalizer 4 is suspended. The carrier recovery circuit 3 corrects a frequency shift in this state. After the frequency shift is corrected by the carrier recovery circuit 3, the FIR equalizer 4 is operated.
With this procedure, however, the carrier recovery circuit 3 operates in the state where the FIR equalizer 4 is suspended. Therefore, the convergence time of the timing recovery circuit 2 and/or the carrier recovery circuit 3 sometimes increases if a distortion of an input signal is large.
As described above, it is difficult in the conventional technology to achieve both of stable demodulation of a distorted input signal, and a reduction in the convergence time of an initial operation.